1. Field of the Invention
The present invention relates to a pulse width modulation control circuit, and more particularly, to a pulse width modulation control circuit applied to charge an output capacitor of a power converter.
2. Description of the Prior Art
In the field of digital cameras, there is sometimes inadequate light for taking pictures. In order to obtain better photographic quality, a flashlight for providing additional light is needed. The working voltage of the lamp in the flashlight, however, is much greater than the camera's internal battery voltage. For example, the working voltage of a flashlight lamp is about 300V, whereas the working voltage of Lithium battery is about 3V-4.2V, and the working voltage of two AA batteries is about 2V-3V. At this time, high-voltage charging circuits are installed inside the cameras, which use a flyback topology and a transformer with large ratio of windings (due to the thinness of the cameras, the ratio of windings is about 10) to charge a high-voltage capacitor in order to attain a higher voltage (up to 300V in general). When the high-voltage capacitor attains the working voltage of the flashlight, it provides the required energy for the flash of the flashlight.
Please refer to FIG. 1. FIG. 1 is a diagram of a charging circuit with flyback topology according to the prior art. As shown in FIG. 1, the average charging current lin is generated when a primary-side current Ip of a transformer 40 is filtered by an input capacitor Cin. When a power switch SW is turned on, the primary-side current Ip of the transformer 40 rises with a slope Vin/Lp. The power switch SW is turned off until the primary-side current Ip rises to Vref1/R42. Lp is the value of the primary-side inductance of the transformer 40. At this time, the energy stored in the primary-side inductance of the transformer 40 is transferred to the secondary-side inductance and charges an output capacitor Co through a Schottky diode Do.
The secondary-side current Is then decreases to zero with a slope Vout/Lsec, where Lsec is the value of the secondary-side inductance of the transformer 40. The drain voltage Vsw of the power switch SW decreases due to the resonance caused by the primary-side inductance of the transformer 40 and stray capacitors. When the drain voltage Vsw drops to the second reference voltage Vref2 (for example, the second reference voltage Vref2 is 1.2V), the power switch is turned on again. This process cycles until the charging of output capacitor Co completes.
Please refer to FIG. 2. FIG. 2 is a timing sequence diagram of the waveforms of the primary-side current, the secondary-side current of the transformer, and the drain voltage of the power switch according to the prior art. As shown in FIG. 2, the secondary-side current Is of the transformer 40 gradually decreases to zero after the power switch SW is turned off. If the drain voltage Vsw of the power switch SW decreases to the second reference voltage Vref2 due to resonance, the power switch SW is turned on again. Therefore, a blank time Tb is generated, and the ratio of Tb to the turn-off time Toff of the power switch SW becomes larger with the increase of drain voltage Vsw (the drain voltage Vsw is approximately proportional to the output voltage Vout). Because the primary-side current Ip is negative during this period of the blank time, the average charging current lin decreases with the increase of the output voltage Vout. Therefore, when the voltage of the output capacitor Co is close to its target value, the average charging current lin becomes smaller, which causes the charging time of the output capacitor Co to lengthen.
For this reason, how to provide a PWM control circuit capable of adaptively adjusting the average charging current has already become one of the problems to be solved by researchers.